Subtle Traps Become New Prey

And 'Subtle' Doesn't Mean Small in the North Sea

When once-rampant drilling activity in a region begins declining and the majors begin losing interest, "it's all drilled up" becomes the common refrain.

A close look at the North Sea, however, suggests the oil finders may be looking for the wrong thing in many instances.

"A key thing in my mind is that 88 percent of the fields discovered are structural traps and only about 5 percent are stratigraphic traps," said Rod Christensen, chief geologist at Canadian-based Oilexco.

"It's no surprise that most companies are looking for structures," he said, "so the majors, when most big bumps are drilled, look for other places with big bumps not drilled, or big anomalies you can see on seismic."

According to Irene Haas, senior vice-president institutional research at Sanders Morris Harris, future discoveries, at least in the UK offshore, are more likely to be the under-explored stratigraphic plays, or combination structural-stratigraphic plays.

She emphasized new fields will be subtle and require better data and more rigorous analysis, which ideally will yield innovative concepts leading to new discoveries.

Image Caption

Figure 1:
Brenda Field, Average Absolute Amplitude
Figures courtesy of Oilexco North Sea

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When once-rampant drilling activity in a region begins declining and the majors begin losing interest, "it's all drilled up" becomes the common refrain.

A close look at the North Sea, however, suggests the oil finders may be looking for the wrong thing in many instances.

"A key thing in my mind is that 88 percent of the fields discovered are structural traps and only about 5 percent are stratigraphic traps," said Rod Christensen, chief geologist at Canadian-based Oilexco.

"It's no surprise that most companies are looking for structures," he said, "so the majors, when most big bumps are drilled, look for other places with big bumps not drilled, or big anomalies you can see on seismic."

According to Irene Haas, senior vice-president institutional research at Sanders Morris Harris, future discoveries, at least in the UK offshore, are more likely to be the under-explored stratigraphic plays, or combination structural-stratigraphic plays.

She emphasized new fields will be subtle and require better data and more rigorous analysis, which ideally will yield innovative concepts leading to new discoveries.

"When you say there are more stratigraphic traps to be found in the North Sea," Christensen said, "we're not on the downside of the exploration parabola showing just small things to be found."

He pointed to the Buzzard discovery in 2001, which is an accumulation of maybe 400 million barrels of recoverable resources. Then there's Oilexco's recent find at the Brenda field with perhaps 200 million barrels recoverable.

"I think if you go in and do AVO-type seismic work looking for stratigraphic traps rather than just using seismic to look for four-way closure on structures," Christensen said, "you're likely to find a lot more stratigraphic traps. It's like in the Gulf of Mexico where structures were drilled first and then new seismic techniques found a lot of stratigraphic accumulations."

Reinforcing this opinion, the Geological Society of London recently sponsored a conference on what was called "the deliberate search for the stratigraphic trap."

"Everyone's realizing we must start looking for these deliberately," Christensen said, "and not find them like we used to, either by mistake or for the wrong reason, such as accidentally while drilling structures."

Tools of the Trade

The Brenda hydrocarbon accumulation and trapping mechanism initially was a head-scratcher given the differing oil/water contacts in neighboring wells that were originally thought to be the same accumulation. Figure 1 and Figure 2

To get a handle on things, Oilexco used the London-office arm of Houston-based GX Technology (GXT) — an approach that's in line with its practice of employing locally-sited companies in the North Sea region, where possible. In turn, GXT brought UK-based rock-physics experts IKON Science on board for the project.

A multi-disciplinary effort ensued to identify potential drilling targets, using both 3-D seismic and data from nearby producing and non-producing wells.

"The focus was on pre-processing of the seismic data, model building and pre-stack depth migration," said Ian Jones, geophysical adviser at GXT.

"We didn't know in advance we were dealing with a stratigraphic occurrence," Jones said, "but one of the relevant details in using the technology we used is that it preserves the amplitude information correctly so we're not destroying the stratigraphic information in processing."

Among the tools used was gridded tomography, where velocities were estimated continuously throughout the volume. Gridded tomography technology employs an auto-picker so the process is fully automated and the manpower can be focused on the Quality Control of what was picked.

Because the process is automated, huge volumes of data are run quickly on a supercomputer cluster.

A pre-processing technique known as SRME (surface-related multiple elimination) was used for multiple suppression.

"A distinguishing feature here is you don't need knowledge of the velocity field to use it," Jones said, "whereas other de-multiple techniques require you to have a good guess at the velocity."

Careful pre-processing to remove noise and multiple contamination, followed up by high fidelity 3-D pre-stack depth-migrated imaging resulted in a data volume suitable for accurate AVO and EI analysis, according to Jones.

"We happened to map a big blob of this AVO anomaly that couldn't be attributed to a structural reason to be there," Christensen said. "It had to be stratigraphic, so based on that and core work we thought that's what it was, but we still had to drill the first well to be sure.

"We also reappraised the structural interpretation based on the stratigraphic evidence," he said, "leading to a correct re-classification of the stratigraphic ties. It was intriguing to see the mis-positioning of some of the old wells based on old-tech imaging, and then to see we were spot-on with the new images."

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